Although thrombocytosis may cause vascular thrombosis with resultant organ or tissue ischemia, the occurrence of coronary thrombosis and myocardial infarction (M) in this circumstance is rare. To our knowledge, this association has been reported only once previously and that in a 22 year-old man. In this report, we describe another 22 year-old man with fatal congestive heart failure nearly a year after a large MI associated with thrombocytosis. Thrombocytosis appears to be the only reasonable cause of the fatal CAD in the above described patient. The presence of thromboemboli in the intramural coronary arteries, in peripheral, splenic, pulmonary and renal arteries, the young age of the patient, the morphologic features of the luminal material which caused the multiple arterial narrowings, i.e., its dissimilarity to atherosclerotic plaques, all strongly support the view that the arterial narrowings in this patient were the result of thrombocytosis.